In a known process for the direct reduction of iron ore, hereinafter referred to as the subject fluidised bed DRI process, iron ore fines are passed downwardly in succession through a series of fluidised bed reactors against an upward counterflow of reducing gas. The process is practised commercially in Venezuela and is known there as the FIOR process. In that plant, the product of the lowermost reactor (the first reactor for the gas flow) is passed to a briquetting facility to produce a stable metallised iron product which is, inter alia, a suitable feed for electric arc steel furnaces, so-called mini-mills. The reducing gas delivered to the lowermost reactor is typically a mix of a reformed natural gas with compressed scrubbed top gas recycled from the uppermost reducing reactor. In each reactor, this gas serves as the fluidising medium as well as the reductant and is therefore delivered to the bottom of each reactor housing via a plenum chamber, for admission to the main reaction chamber through multiple nozzles of a distributor plate which supports the fluidised bed. Within each reactor, internal cyclones extract fine ore dust from the top gas before it is delivered to the next upper reactor or, in the case of the uppermost reactor, recycled to the reducing gas feed stream. The dust extracted in the cyclones is returned to the fluidised bed via respective diplegs.
Particular forms or modifications of this process are described, for example, in U.S. Pat. Nos. 5,082,251, 5,370,727 and 5,439,504, international patent publication WO 96/10094 and Australian patent application no. 15007/95.
In the current commercial practice of the FIOR DRI process, the gradual build up of accretions at various points in the reactor configuration steadily diminishes the performance of the process and eventually reduces gas flows to a point where the process must be stopped and the accretions removed. Experience has shown that accreted material in nozzles and in the reactor lower plenum chamber tends to be deposited in regions of stagnation in the fluid flow. Accretions occur in stagnant regions resulting from direct gas impingement as well as in regions of flow separation.
The problem was addressed in Australian patent 490892, which proposed a modified shape of nozzle for the distributor plate. Although not directly suggested as relevant to the reduction of the rate of accretion formation, this reference also illustrates a dual baffle plate arrangement in the plenum chamber under the distributor plate rather than the single baffle plate used in the FIOR reactors. The upper baffle plate is an annulus or donut, and the other is a smaller disc.
More complex distributor plate configurations are described in British patent 1119250, Swiss patent 583066, and European patent publication 421506. The first two of these disclose a double-plate arrangement with offset apertures. EP 421506 depicts two spaced perforated plates or screens sandwiching a bed of balls.
It is an object of the present invention, in a preferred application, to reduce the rate at which accretions form in the subject fluidised bed DRI process at and adjacent the distributor plate nozzles of the inlet plenum assemblies.